December 6, 2000: This ghostly apparition is actually an interstellar cloud caught in the process of destruction by strong radiation from a nearby hot star. This haunting picture, snapped by the Hubble telescope, shows a cloud illuminated by light from the bright star Merope. Located in the Pleiades star cluster, the cloud is called IC 349 or Barnard's Merope Nebula.
The cloud has been shaped by its closeness to Merope. The distance between the two objects is about 3,500 times the separation of the Earth from the Sun (about 0.06 light-year). The cloud, which has been drifting through the Pleiades star cluster, is moving closer to Merope at a speed of about 6.8 miles per second (11 kilometers per second). Astronomers have proposed that the strong starlight shining on the dust in the cloud decelerates the dust particles. Physicists call this phenomenon "radiation pressure."
Smaller dust particles are slowed down more by the radiation pressure than the larger particles. Thus, as the cloud approaches the star, there is a sifting of particles by size, much like grain thrown in the air to separate wheat from chaff. The nearly straight lines pointing toward Merope are thus streams of larger particles, continuing on toward the star while the smaller decelerated particles are left behind at the lower left of the picture.
Merope is just outside the picture on the upper right. The colorful rays of light at the upper right, pointing back to the star, are an optical phenomenon produced within the telescope, and are not real. However, the remarkable parallel wisps extending from lower left to upper right are real features, revealed for the first time through Hubble's sharp "eyes."
Over the next few thousand years, the nebula if it survives the close passage without being completely destroyed will move on past Merope, somewhat like a comet swinging past our Sun. This chance collision allows astronomers to study interstellar material under very rare conditions and thus learn more about the structure of the dust lying between the stars.